In the advancing information society, the leakage of confidential information has become a serious problem, and importance of techniques for preventing the information leakage is increasing. With regard to the information leakage prevention technique, a technique has been developed for encrypting, for example in digital data, the data so that if a third party obtains it, cannot find its contents. The encryption technique has already been used as effective means to prevent the information leakage of digital data.
On the other hand, the technique for preventing the information leakage of a printed matter printed on a paper medium and the like has not been established sufficiently yet, and there has been no example of practical application. Some statistics show that about half of information leakage occur with printed matters, so for printed matters, as well as digital data, there is an urgent need for the development of a technique for preventing information leakage. Specific examples of printed matters for which a measure to prevent information leakage is needed are, an invoice of purchase of goods, a bill of a credit card and the like, medical records in a hospital, record cards and name lists in a school, and so on.
According to PCT application JP/2007/000215 (hereinafter, referred to as patent document 1), not only a digital image, but also an image printed on paper can be encrypted to prevent information leakage. Meanwhile, medical records in a hospital and bills and the like can be defined as a type of visual information. Therefore, in the description of the present invention (in this description), they are collectively referred to as an “image”.
The outline of the image encryption disclosed in patent document 1 is described.
In an input image 10 illustrated in FIG. 1(A), a part of it is specified as an encryption region 11, and an image processing is performed for the image in the encryption region 11 in accordance with a password (encryption key). As a result, an encrypted image 20 illustrated in FIG. 1(B) is generated. In the encrypted image 20, the image in the encryption region 11 described above has been converted into an image (converted image) in which the original contents cannot be recognized.
A restoration method of the encrypted image 20 is illustrated in FIG. 2.
When a correct password (decryption key) is input for the encrypted image illustrated in FIG. 2(A), the original image 10 is restored as illustrated in FIG. 2(B). However, when an incorrect password is input for the encrypted image 20, the converted image 21 is not decrypted to be the original image, and converted into a different image 31, as illustrated in FIG. 2(C). Thus, when an incorrect password is input for the encrypted image 20, the original image 10 is not restored.
However, in the case of the image encryption technique in patent document 1, there is a disadvantage as illustrated in FIG. 3.
That is, even when an incorrect password is input for the encrypted image 20 illustrated in FIG. 3(A), there is a possibility that the converted image 21 is coincidentally converted into an image 41 that is similar to the original image as illustrated in FIG. 3(B), and allowing the concealed contents to be inferred.
This is not preferable in terms of security. Therefore, there is a need for a mechanism with which, when an incorrect password is input, a converted image is not decoded.
Meanwhile, a conventional art related to the present invention, for example, Japanese Laid-open Patent Application 2006-332826 (hereinafter, referred to as patent document 2), is disclosed. In a method of patent document 2, when performing transmission/reception of a material by FAX, the transmitter selects a region in the material that needs to be confidential, and specifies an encryption password. By this, the set encryption password is overlapped with and printed on the cover sheet, and these materials are transmitted to the receiver side by using communication means such as FAX. At the receiver side, the password is extracted from the cover sheet of the received material, and when it matches the password input by the receiver, the text of the document is printed in a state in which the receiver can read it. In addition, according to patent document 2, the embedding of the password information into the cover sheet is to be done using an electronic watermark or a barcode.
However, the conventional arts such as the one described above have a problem as follows to be solved.
According to the method of patent document 2, password information is embedded into a document image using an electronic watermark or a barcode. However, since the electronic watermark is a system for embedding information by slightly changing pixel values so as not to lose the value of the image contents, it is susceptible to distortion caused with printing, copying and scanning, and there is a high possibility that it does not work at the time of authentication. In addition, a barcode provides an embedding method that is resistant to distortion, but it cannot be applied when there is no appropriate margin region besides the encryption region, and when there are a plurality of regions to be encrypted, barcodes needs to be printed additionally, the number of barcodes corresponding to the number of regions.
As illustrated in FIG. 4(A), when there are three encryption regions 51, 52, 53 in an original image 50, an individual password is given to the encryption regions 51-53 respectively. The encryption regions 51-53 are converted into images 61-63 on the basis of the passwords assigned respectively, generating an encrypted image 60 as illustrated in FIG. 4(B). At this time, three barcodes 66-68 representing the respective pieces of information about the three passwords above (password information) are embedded in the upper area of the encrypted image 60.
In the case of the method of patent document 2, the encryption region and barcode need to be linked, requiring elaboration for that. For this reason, there are problems in practicality, in terms of an increase in the toner cost due to the addition of the barcode, increase in the processing amount for associating the encryption area and the barcode, and so on.
Patent document 1: PCT/JP2007/000215
Patent document 2: Japanese Laid-open Patent application No. 2006-332826 in the Japan Patent Office
Patent document 3: Japanese Patent Application No. 2006-266015 in the Japan Patent Office